The ecology of Bacillus thuringiensis, a common biopesticide, is poorly understood. Our main objective was to investigate the ecology of this bacteria on the phylloplane. In a first study 35 leaf samples of the genus Piper were collected from the Colombian Andean forest. Two hundred and fifty-six isolates of B. thuringiensis were obtained from 74% of the samples studied. The isolates were characterized by crystal morphology, the presence of cry genes by PCR and toxicity against insects. The populations of viable vegetative cells and spores per unit area were estimated (2-5x103 cfu/cm2 of leaf). Overall, no significant differences in the number of B. thuringiensis isolates per cm2 of leaf nor in the B. thuringiensis characteristics were found among the different Piper species evaluated. After observing B. thuringiensis on the phylloplane, a comparison was performed between soil and leaf populations. B. thuringiensis was isolated from the phylloplane and soil in plantings of maize and bean from Colombia; and 214 isolates were recovered from 96 phylloplane samples (0-34 cfu/cm2) while 59 isolates from 24 soil samples. All the isolates were characterized as above-mentioned. The predominant population of B. thuringiensis on the phylloplane harbored cry1 gene and was active against Spodoptera frugiperda, whereas in soil the isolates harboring cry11 gene and active against Culex quinquefasciatus predominated. The predominance of specific B. thuringiensis populations both on the leaves and in the soil, suggests the presence of differential selection in B. thuringiensis populations on the phylloplane and in soil. Then, we addressed the process of immigration of B. thuringiensis from soil to leaves. Two different B. thuringiensis strains were inoculated into soils, onto seeds or onto lower leaves of bean plants to determine if they were able to disperse to upper leaves under controlled conditions. While B. thuringiensis isolates were commonly recovered from leaves exposed to such inocula, populations were very low (less than 10 cfu/cm2 of leaf). In addition, the number of cells of B. thuringiensis recovered decreased with increasing distance from the soil or from the inoculated leaves. This indicates that B. thuringiensis disperses poorly from the soil or the seed to the leaves or between leaves of the same plant under controlled conditions. Moreover, the ability of several B. thuringiensis strains to colonize plant surfaces was assessed and compared with that of more common epiphytic bacteria. While all B. thuringiensis strains multiplied to some extent after inoculation on bean plants, their maximum epiphytic population sizes of 106 cfu/g of leaf were always much less than that achieved by other resident epiphytic bacteria or an epiphytically fit Pseudomonas fluorescens strain. Many cells were in a spore form soon after inoculation onto plants. The growth of B. thuringiensis was not affected by the presence of Pseudomonas syringae spp. when co-inoculated, and vice versa. B. thuringiensis strains harboring a green fluorescent protein marker gene did not form large cell aggregates, were not associated with other epiphytic bacteria, and were not found associated with leaf structures when directly observed on bean leaves by epifluorescent microscopy. Finally, we analyzed the capacity of B. thuringiensis to grow on a medium designed to simulate the nutrient composition of the phylloplane but the growth observed was very poor compared to other bacteria. This bacterium apparently has greater nutrient requirements than other bacterial species that are prominent inhabitants of the phylloplane.